I. Field of the Invention
The present invention relates to a ceramic composition of a high dielectric constant which essentially consists of barium titanate and has a ceramic microstructure, and which is primarily used for a ceramic capacitor.
II. Description of the Prior Art
It is conventionally known that barium titanate as a major component have been used as materials for various ceramic capacitors. As a matter of fact, barium titanate has ferroelectric characteristics. More particularly, isometric perovskite barium titanate is obtained at a high temperature; tetragonal barium titanate where the crystal is slightly elongated along the C-axis is obtained at a temperature below 120.degree. C.; orthorhombic barium titanate is obtained at a temperature of about 0.degree. C.; and rhombic barium titanate is obtained at a temperature of about -80.degree. C. The phase transition point at the temperature of about 120.degree. C. is especially called the Curie point. Barium titanate shows paraelectric characteristics at a higher temperature than the Curie point, whereas it shows ferroelectric characteristics at a lower temperature than the Curie point. The dielectric constant of barium titanate is as high as 10,000 at the Curie point. It is thus noted that barium titanate by itself does not have a high dielectric constant at room temperature. Various compact capacitors are commercially available in which the high dielectric constant of barium titanate at about the Curie point is obtained at a temperature lower than this point so as to allow the capacitor to have a proper (electrostatic) capacitance at room temperature. The peak value of the dielectric constant at a high temperature can be shifted by additives so as to obtain the peak value at a lower temperature. These additives are called a shifter and are generally selected from a stannate such as BaSnO.sub.3, SrSnO.sub.3, CaSnO.sub.3, PbSnO.sub.3, CuSnO.sub.3, ZnSnO.sub.3 and CdSnO.sub.3 ; a zirconate such as BaZrO.sub.3, CaZrO.sub.3 and SrZrO.sub.3 ; and a titanate such as SrTiO.sub.3 and PbTiO.sub.3. The effect of the shifter is greatest for a stannate, followed by a zirconate and a titanate, in that order.
A barium titanate compound containing a shifter has been used for a ceramic capacitor of a single sheet lead type. Meanwhile, a laminate chip manufacturing technique has been recently developed, so that a dielectric sheet of 30 to 100 .mu.m thickness can be easily obtained. A laminate ceramic chip capacitor is then developed having a structure in which a set of thin dielectric sheets sandwiched between electrodes is laminated together. The laminate ceramic chip capacitors have been widely used in various types of electronic equipment. In this manner, the conventional dielectric ceramic composition is often utilized as a thin laminate dielectric sheet. However, the laminate ceramic chip capacitor has a dielectric of 10 to 20 .mu.m thickness, whereas the conventional single-sheet ceramic capacitor has a dielectric of 100 to 10,000 .mu.m thickness. As a result, the laminate ceramic chip capacitor is adversely affected by the intensity of the electric field about 5 to 10 times as much as is the conventional capacitor. For this reason, a strong demand has arised for a composition which is less dependent on voltage. Furthermore, when the dielectric layer is thin, crystal defects of the ceramic material tend to affect electrical characteristics of the capacitor. As a result, the ceramic material must have a uniform grain size and a small number of pores each having a small diameter.
In order to meet the above-mentioned demands, inventors of the present invention have proposed a ceramic composition which has a high dielectric constant, a small number of crystal defects, little dependency on voltage, and a high breakdown voltage. This ceramic composition of a high dielectric constant essentially consists of 100 mol parts of barium metatitanate (BaTiO.sub.3), (2/3)(7 .+-.1) mol parts of cerium dioxide (CeO.sub.2), and 7.+-.1 mol parts of titanium dioxide (TiO.sub.2).
This ceramic composition of a high dielectric constant has a ceramic microstructure in which a grain size and a pore diameter are small. At the same time, the ceramic composition has a high dielectric constant, a high breakdown voltage and little dependency on voltage. However, the above ceramic composition has a small disadvantage in that the composition has a relatively low insulation resistance, and the electrodes are visible through the thin dielectric layers, since the ceramic composition is vivid red, when the thin dielectric layer is alternately laminated with the electrode.